Application.Match on multiple columns - excel

Question

I have a working macro that finds duplicates on a different worksheet and highlights them.
Sub dedup()
Dim i As Long
Dim x As Variant
With Worksheets(1)
For i = 2 To .Cells(.Rows.Count, "C").End(xlUp).Row
x = Application.Match(.Cells(i, "C").Value, Worksheets(2).Columns("C"), 0)
If Not IsError(x) Then
.Cells(i, "C").Interior.ColorIndex = 4
End If
Next i
End With
End Sub
I am trying to now filter the "duplicates" based on a criteria, that is, another column - Column A. The duplicates are to be considered duplicates only if even the Column A values match.
Sub dedup()
Dim i As Long
Dim x As Variant
With Worksheets(1)
For i = 2 To .Cells(.Rows.Count, "C").End(xlUp).Row
x = Application.Match(.Cells(i, "C").Value & .Cells(i, "A").Value, Worksheets(2).Columns("C") & Worksheets(2).Columns("A"), 0)
If Not IsError(x) Then
.Cells(i, "C").Interior.ColorIndex = 4
End If
Next i
End With
End Sub
The logic here, was to concatenate my query and the range I am querying. But, I get a "Type mismatch" error.
Column C is a numeric value, and Column A is a date value

Answer 1

Hi see if following code answers your expectations. I have tweaked it a little bit as per what I could read from your question.
Sub dedup()
Dim i As Long
Dim x, y As Variant
With Worksheets(1)
For i = 2 To .Cells(.Rows.Count, "C").End(xlUp).Row
x = Application.Match(.Cells(i, "A").Value, Worksheets(2).Columns("A"), 0)
y = Application.Match(.Cells(i, "C").Value, Worksheets(2).Columns("C"), 0)
If Not IsError(x) Or IsError(y) Then
.Cells(i, "C").Interior.ColorIndex = 4
End If
Next i
End With
End Sub

Answer 2

You cannot concatenate multicell ranges like Worksheets(2).Columns("C") & Worksheets(2).Columns("A"). Remember this is vba - not python :). You can, however do it within excel as in an array formula so you can use something like
Dim sShtName as String: sShtName = Sheets(2).name
Evaluate("=MATCH(""" & .Cells(i, "C").Value & .Cells(i, "A").Value & """," & sShtName & "!C:C&" & sShtName & "!A:A,0)")
Don't expect this to be fast. You can build a loop to test a match. You can also add a column in Sheets(2) that will hold the concatenated values and Match in this column.
Edit
A much better approach (as suggested by #BigBen) is the use of Application.CountIfs function. The reason it is better (#BigBen can comment on this) is that you do not have to check for errors and all you want is to check whether the value exists or not (i.e. you don't care about where that value is)